In charged particle beam systems, such as electron microscopes or focused ion beam (FIB) systems, a source generates charged particles which are then focused by an optical column into a beam and directed onto the surface of a target to be imaged and/or processed. In the column, this beam may be blanked, that is, diverted into a stop to turn it off, deflected to move it around on the target surface.
A beam having a lower beam current, that is, fewer charged particles, can typically be focused to a smaller diameter than a beam having a greater current. Using a lower beam current can therefore provide higher resolution imaging or processing. A lower beam current also results in less damage to the target.
An ion beam can be used to mill or sputter material in controlled patterns from the surface of a target. The milling rate is roughly proportional to the beam current. Higher beam currents are therefore preferred when it is desired to quickly remove material, although the higher beam current typically results in lower resolution processing. Processing will sometimes use a two-step processing, with a high current beam to remove material quickly, and then a lower current beam to more precisely complete the milling
Although an ideal beam would have all the ions uniformly distributed within a desired beam diameter, in actuality, the beam current distribution is more or less bell-shaped and tapers off away from the beam center. This “tail” reduces image resolution and makes it impossible to mill a straight vertical edge.
Some applications require imaging, coarse milling, and fine milling. In particular, when a milled pattern needs to be precisely located with respect to a pre-existing feature on a target, it is necessary to first image the target with a lower current FIB, and then to switch to a higher current (typically larger diameter) FIB for coarse milling and then a lower current beam for fine milling. One important example of such an imaging/milling process is the preparation of thin “lamellae” (singular: “lamella”) of various types of samples, such as semiconductor devices and cryo-frozen biological samples. In the case of semiconductor device failure analysis, a particular region of interest (RoI) within an integrated circuit, usually containing a defective device to be analyzed, is exposed by FIB milling on both sides, leaving a thin slice (lamella) of material remaining which contains the defective device—these lamellae are thin enough for use in high voltage transmission electron microscopes (TEMs) or scanning transmission electron microscopes (STEMs) where atomic resolutions are, in principle, available. Because the lamellae are only tens of nanometers thick, and the defects being observed may be on the scale of nanometers, the milling to create the lamella is extremely precise.
During preparation of a lamella, it is necessary to switch between using large current, large diameter beams suitable for rapid milling, lower current, smaller diameter beam for fine milling, and even lower current, smaller diameter beams imaging. This is typically done by changing a beam-defining aperture (BDA) through which the beam passes. BDAs are typically holes in a metal strip, allowing only charged particles that pass through the hole to form the beam. There are typically several BDAs, or holes, in a metal strip, and switching apertures typically entails moving the strip so that a hole of a different diameter is positioned in the path of the beam.
FIG. 1 is a schematic cross-sectional view of a prior art aperture 100. A hole 110 is formed in a silicon substrate 108 of roughly the size of the desired aperture hole. Layers may be deposited on the silicon substrate such as a SiO2 layer 106 and a SixNy layer 104 to aid in the formation of this hole. A molybdenum layer 102 is then deposited on all surfaces conformally to protect the aperture from the ions in the ion beam. The layer is thin, typically 200-500 nm, to avoid substantially reducing the aperture hole diameter. Beam defining apertures in ion beam systems have a limited life because the ions in the ion beam impinge on the aperture structure, eroding it and expanding the hole.